In a magnetic disk or tape data storage device, data is commonly stored on a magnetic medium by saturation recording in which each portion of the medium is magnetized to the point of saturation in one of two directions. The data to be stored is typically encoded to satisfy certain constraints and the encoded data is used to modulate the direction of magnetization. In a coded representation known as NRZI, each "one" bit of the encoded data causes a transition in the direction of magnetization, while each "zero" bit of the encoded data causes the magnetization direction to remain unchanged. A clock signal is used to write a sequence of encoded NRZI bits as a recording head moves along a track on the medium such that one bit is written at each clock tick.
When a read head is passed over the recorded data track, a voltage pulse is produced at each transition in magnetization. Successive voltage pulses have opposite polarity since successive magnetic transitions are in opposite directions. The written NRZI data sequence may be reconstructed from the resulting voltage waveform by associating a "one" bit with every clock tick at which a pulse occurs and a "zero" bit with every clock tick at which no pulse occurs. The original user data may then be decoded from the NRZI data.
Similar waveforms composed of pulses sometimes occur in other applications, including data storage on optical media and data communications via modem, network, radio, or fiber-optic link.
To recover the written or transmitted data sequence, the receiver requires a clock signal synchronized with the received waveform. At each tick of this synchronized clock signal the receiver or read circuitry generates one bit of the NRZI data sequence by processing the surrounding waveform. It is often impossible or at least undesirable to store or transmit a separate synchronized clock signal with the data waveform. Instead, constraints are applied to the encoded NRZI data sequence to ensure that timing information may be extracted from the data waveform itself and used to "recover" a synchronized clock signal. Such a system is referred to as "self clocking".
Prior art magnetic disk drives typically use a data detection system known as peak detection, in which analog circuitry determines the time instant of each peak in the waveform. Peaks that meet a qualification requirement, such as exceeding a threshold amplitude, are considered to have been caused by a magnetic transition and are therefore associated with an NRZI "one" bit. The recovered clock divides the waveform into bit cells or windows where the nominal position of a peak is at the center of a window. A "one" bit is output for each window that contains a qualified peak and a "zero" bit is output for each other window. The phase and frequency of the recovered clock are adjusted in a phase locked loop so as to keep the windows properly aligned with the peaks.
Inter-symbol interference (ISI) occurs when pulses are placed so close together in a waveform that they overlap significantly. The position of a peak in a waveform can be shifted by ISI from other nearby pulses. Thus the recording density of a peak detection channel is limited by the fact that ISI must not be permitted to move a peak from one window to another. Peak shift due to ISI also causes apparent timing errors that affect the performance of the clock recovery circuit.
To achieve higher recording densities and/or greater noise immunity than peak detection channels, another class of detection methods is based on amplitude sampling of the signal waveform. Many such sampling detection methods can be implemented using either analog or digital signal processing devices. In either case, a synchronized clock signal is required to control the sample timing and a means must be provided to recover such a clock signal from the data waveform. Thus there is a need for new timing recovery methods suitable for use with sampled amplitude detectors at high recording densities. The present invention meets this and other needs.
This Application is related to U.S. Pat. No. 5,291,499, and application Ser. No. 07/879,938, filed May 8, 1992, of Richard T. Behrens, Trent Dudley, and Neal Glover, entitled "Digital Pulse Detector", both owned by the same entity. Each of these applications is incorporated herein by reference for all that is disclosed and taught therein.